Each reference cited herein is expressly incorporated herein by reference in its entirety for all purposes.
Analog-to-Digital Converters, or ADCs, are essential components that convert data from analog sensors and receivers to the digital domain. Most commercial ADCs are based on semiconductor transistors and operate near room temperature. For certain applications, newer superconducting ADCs that operate at cryogenic temperatures near 4 K have been developed. See, for example, the following U.S. Pat. Nos. 3,983,419; 4,082,991; 4,393,357; 4,586,010; 4,694,276; 4,837,604; 4,904,882; 4,943,556; 4,956,642; 4,962,086; 4,977,402; 4,983,971; 4,990,462; 5,021,658; 5,064,809; 5,075,253; 5,171,732; 5,189,420; 5,198,815; 5,252,294; 5,345,114; 5,347,086; 5,347,143; 5,399,881; 5,400,026; 5,455,511; 5,536,947; 5,543,988; 5,550,389; 5,619,139; 5,629,838; 5,680,018; 5,687,112; 5,773,875; 5,780,314; 5,863,868; 5,869,958; 5,878,334; 5,892,243; 5,900,618; 5,912,503; 5,916,848; 5,939,730; 5,992,354; 6,000,225; 6,051,440; 6,066,600; 6,105,381; 6,115,233; 6,157,329; 6,166,317; 6,235,067; 6,284,586; 6,301,330; 6,313,587; 6,329,139; 6,331,805; 6,348,699; 6,365,912; 6,387,329; 6,439,253; 6,453,264; 6,486,756; 6,495,854; 6,509,853; 6,570,224; 6,608,518; 6,608,581; 6,610,367; 6,617,987; 6,649,929; 6,653,962; 6,710,343; 6,728,113; 6,759,010; 6,759,974; 6,771,201; 6,798,083; 6,916,719; 6,949,887; 6,962,823; 6,980,142; 7,019,391; 7,034,660; 7,038,604; 7,075,467; 7,084,691; 7,133,375; 7,144,553; 7,151,209; 7,180,074; 7,224,041; 7,233,144; 7,259,373; 7,272,431; 7,280,623; 7,289,197; 7,289,312; 7,330,369; 7,362,125; 7,365,663; 7,375,417; 7,395,166; 7,403,580; 7,436,910; 7,436,911; 7,436,912; 7,440,490; 7,471,224; 7,488,960; 7,489,537; 7,489,745; 7,495,242; 7,495,244; 7,495,245; 7,495,592; 7,496,158; 7,498,897; 7,501,644; 7,501,877; 7,507,960; 7,511,496; U.S. Pat. Nos. 7,535,005; 7,554,369; 7,560,932; 7,598,897; 7,630,227; 7,659,526; 7,659,981; 7,680,474; 7,687,409; 7,692,270; 7,701,220; 7,719,280; 7,719,392; 7,719,453; 7,728,748; 7,733,253; 7,864,560; 7,869,221; 7,869,974; 7,875,876; 7,876,869; 7,917,798; 7,928,875; 7,944,253; 7,956,640; 7,982,646; 7,986,218; 7,991,013; 7,993,813; 8,022,854; 8,031,510; 8,045,364; 8,045,660; 8,050,648; 8,055,235; 8,055,318; 8,073,631; 8,076,249; 8,078,130; 8,081,946; 8,084,762; 8,093,900; 8,130,880; 8,149,894; 8,159,825; 8,169,081; 8,179,133; 8,184,673; 8,188,901; 8,198,621; 8,217,381; 8,249,129; 8,249,540; 8,260,143; 8,260,144; 8,260,145; 8,274,817; 8,278,027; 8,301,104; 8,324,897; 8,390,100; 8,399,365; 8,401,050; 8,401,509; 8,401,600; 8,406,834; 8,416,109; 8,423,103; 8,423,297; 8,441,154; 8,450,716; 8,462,889; 8,466,583; 8,493,771; 8,509,354; 8,509,368; 8,514,986; 8,521,117; 8,565,345; 8,587,915; 8,593,141; 8,604,791; 8,618,799; 8,648,287; 8,653,497; 8,658,994; 8,664,767; 8,664,955; 8,698,570; 8,729,524; 8,736,452; 8,744,541; 8,754,396; 8,787,873; 8,804,358; 8,811,536; 8,867,931; 8,872,690; 8,895,913; 8,901,778; 8,901,779; 8,901,928; 8,904,809; 8,907,531; 8,912,687; 8,912,805; 8,922,066; 8,928,276; 8,933,520; 8,933,594; 8,933,695; 8,937,255; 8,946,938; 8,953,950; 8,957,549; 8,970,217; 8,977,223; 9,019,679; 9,020,079; 9,020,362; 9,036,319; 9,054,094; 9,065,423; 9,065,452; 9,077,412; 9,097,769; 9,105,555; 9,106,203; 9,110,249; 9,154,172; 9,165,979; 9,166,731; 9,213,085; 9,225,918; 9,252,825; 9,261,573; 9,276,615; 9,312,760; 9,312,878; 9,312,895; 9,324,733; 9,344,069; 9,367,288; 9,373,592; 9,392,957; 9,395,425; 9,400,127; 9,400,214; 9,401,240; 9,425,838; 9,442,066; 9,450,696; 9,453,814; 9,464,350; 9,509,315; 9,515,025; 9,520,180; 9,548,878; 9,554,303; 9,554,738; 9,565,385; 9,577,690; 9,588,191; 9,589,686; 9,602,168; 9,608,672; 9,614,532; 9,618,591; 9,647,194; 9,661,596; 9,693,694; 9,696,397; 9,698,607; 9,705,571; 9,742,429; 9,748,937; RE37259; RE44097; 20020060635; 20020154029; 20020177769; 20030076251; 20030179831; 20040022332; 20040120299; 20040195512; 20040217748; 20040217822; 20060017488; 20060145750; 20060170535; 20060197943; 20070055133; 20070075752; 20070077906; 20070081611; 20070098058; 20070194225; 20070223936; 20070293160; 20080048902; 20080049885; 20080101444; 20080101501; 20080101503; 20080107213; 20080186064; 20080252293; 20090073017; 20090140739; 20090153381; 20090168286; 20090232191; 20090232507; 20090232510; 20090265112; 20100026537; 20100026538; 20100057653; 20100066576; 20100149011; 20100259261; 20110054236; 20110109310; 20110210811; 20110288823; 20120062230; 20120062345; 20120082283; 20120112531; 20120112532; 20120112534; 20120112535; 20120112536; 20120112538; 20120112691; 20120119569; 20120119575; 20120119576; 20120119698; 20120123693; 20120157321; 20120166117; 20120184338; 20120198591; 20120213531; 20120223709; 20120228952; 20120228953; 20120228954; 20120235500; 20120235501; 20120235502; 20120235503; 20120235504; 20120235566; 20120235567; 20120235633; 20120235634; 20120239117; 20120242159; 20120242225; 20120244290; 20120248886; 20120248887; 20120248888; 20120248981; 20120256494; 20120274494; 20120328301; 20130004180; 20130036078; 20130253302; 20130272453; 20130315597; 20140013724; 20140027638; 20140097846; 20140113828; 20140166868; 20140199490; 20140233942; 20140266202; 20140285198; 20140286465; 20150061404; 20150069831; 20150078290; 20150125155; 20150137830; 20150143817; 20150219732; 20150229343; 20150236546; 20150255994; 20150333536; 20150338478; 20160028402; 20160028403; 20160033597; 20160087687; 20160091578; 20160097718; 20160161550; 20160197628; 20160245852; 20160267032; 20160283197; 20160292586; 20160292587; 20160324438; 20170026175; 20170134091; 20170163301; 20170176623; 20170179973; 20170244450; 20170244453; 20170244454; 20170244455; and 20170265158; See also Mukhanov, “Superconductor Analog-to-Digital Converters”, Proceedings of the IEEE, vol. 92, p. 1564, 2004.
Superconducting ADCs typically use integrated circuits with many Josephson junctions, and are based on single-flux-quantum (SFQ) pulses. These fast pulses, typically 2 ps wide and 1 mV high, are naturally generated by a Josephson junction biased above the critical current Ic (see FIG. 1), and are responsible for the high speed, low power, and sensitivity of these ADCs. Josephson junctions biased below Ic can regenerate and transmit SFQ pulses, forming an active Josephson transmission line (JTL) that provides the basis of rapid-single-flux-quantum (RSFQ) digital logic (see FIG. 2). These ADCs typically have sampling rates in excess of 20 GHz, and can digitize rapidly varying radio signals with bandwidths in excess of 10 GHz. The ADCs can be integrated with RSFQ digital signal processing on the same chip. In general, superconducting ADCs offers the best combination of broad bandwidth, low power, and high sensitivity of any electronic technology.
One type of prior-art superconducting ADC is known as a phase-modulation-demodulation ADC (PMD). In this system, as shown in FIG. 3 (taken from Mukhanov 2004), an analog electrical input modulates a periodic pulse train of SFQ pulses, either advancing or retarding a given pulse. This asynchronous pulse train is subsequently synchronized, integrated, and averaged, in order to generate additional effective bits. The concept of a PMD ADC has also been embodied in other technologies. For example, Tanoni, U.S. Pat. No. 9,356,704, discloses an analog electrical signal modulating an optical pulse train, which is subsequently demodulated to generate electrical bits.
While radio waves are widely used for propagating broadband signals in free space, an alternative mode for long-distance communication is via light on optical fibers. The signals can propagate for long distances with little attenuation. Typical optical carriers are infrared light with wavelength 1.2-1.7 μm. See en.wikipedia.org/wiki/Fiber-optic_communication. Optical signals may also be used for imaging and for intra-computer and inter-computer communication.
In most cases, an electrical signal is modulated onto the optical carrier at the transmission end, and demodulated at the receiving end. A wide variety of technologies can be used for modulators and transducers that operate near room temperature, including electro-optic, magneto-optic, acousto-optic, photoelastic, and electro-absorptive effects. See, for example, www.rp-photonics.com/optical_modulators.html.
Because of the extremely high optical frequencies, many broadband multi-GHz signals can be carried on the same optical fiber, using a technology known as Wavelength-Dispersive Multiplexing (WDM). See, for example, en.wikipedia.org/wiki/Wavelength-division multiplexing. In some cases, a set of integrated micro-ring waveguides can be used as add-drop multiplexers, to consolidate or split off the various component wavelengths. See for example, U.S. Pat. Nos. 7,539,418; 8,805,130; 2015/0168748; also Q. Xu, et al., “Cascaded silicon micro-ring modulators for WDM optical interconnection”, Optics Express, vol. 14, p. 9431, 2006, from which FIG. 4 was copied.
There have been several approaches in the prior art to digitizing such optical signals. One approach is to demodulate the optical signal to generate a radio-frequency electrical signal, and then use an electrical ADC to generate a digital representation. However, a system that integrates these functions together should be more efficient and compact. U.S. Pat. No. 5,850,195 provides a monolithic light-to-digital signal converter. Other patents for broadband optical digitizers include U.S. Pat. Nos. 6,265,999; 7,564,387; 8,514,115; 8,725,004; 8,730,562.
These optical ADCs should be distinguished from optically-enhanced electrical ADCs, where the input and output signals are electronic, but optical elements are used in part of the sampling or quantizing. Low-jitter optical clocks may be used, or precision optical delay lines. For example, U.S. Pat. No. 6,771,201, Hybrid Photonic Analog-to-Digital Converter, discloses a system whereby a train of optical pulses are used to generate a train of fast electrical pulses using superconducting devices, which are then used as a sampling clock for a superconducting ADC. But the signal to be quantized is an electrical signal, not an optical signal. Other optically-enhanced electrical ADCs (which have sometimes been labeled “photonic ADCs”) are disclosed in the following U.S. Pat. Nos. 7,876,246; 6,100,831; 6,661,361; and 6,700,517. See also: U.S. Pat. Nos. 3,999,063; 4,078,232; 4,209,853; 4,294,127; 4,320,484; 4,502,037; 4,712,089; 4,770,483; 4,851,840; 4,926,177; 4,928,007; 5,097,473; 5,101,270; 5,264,849; 5,267,139; 5,381,147; 5,403,040; 5,552,881; 5,583,950; 5,627,920; 5,636,050; 5,892,151; 5,982,932; 6,064,507; 6,118,397; 6,175,320; 6,188,342; 6,326,910; 6,404,365; 6,404,366; 6,420,984; 6,420,985; 6,434,173; 6,469,649; 6,469,817; 6,525,682; 6,529,150; 6,636,681; 6,671,298; 6,686,997; 6,713,224; 6,714,149; 6,754,631; 6,771,201; 6,784,466; 6,873,468; 6,956,653; 7,016,421; 7,050,182; 7,083,998; 7,124,036; 7,194,139; 7,212,140; 7,233,261; 7,245,795; 7,294,446; 7,327,913; 7,350,939; 7,362,931; 7,397,979; 7,400,703; 7,420,505; 7,483,600; 7,564,387; 7,570,184; 7,715,720; 7,787,767; 7,801,395; 7,858,949; 7,940,201; 7,956,788; 7,967,764; 7,990,299; 8,026,837; 8,126,298; 8,263,928; 8,269,658; 8,334,797; 8,384,978; 8,432,153; 8,442,402; 8,446,305; 8,456,336; 8,466,819; 8,514,115; 8,548,331; 8,593,716; 8,618,966; 8,655,176; 8,686,712; 8,692,774; 8,725,004; 8,779,955; 8,836,703; 8,886,726; 8,902,095; 8,902,096; 8,928,510; 8,953,950; 8,954,554; 8,963,751; 8,965,211; 9,001,619; 9,045,970; 9,176,361; 9,197,471; 9,201,287; 9,329,413; 9,341,921; 9,350,458; 9,389,326; 9,395,456; 9,413,372; 9,438,263; 9,442,205; 9,450,597; 9,450,696; 9,467,223; 9,502,856; 9,557,433; 9,571,731; 9,612,304; 9,645,377; 9,647,827; 9,716,553; 9,734,285; 9,746,743; 9,772,414; RE28954; RE35766; 20020067299; 20020163454; 20040001016; 20040096143; 20060072186; 20070110362; 20070140613; 20070274733; 20080088502; 20090142051; 20100002281; 20100201345; 20100277354; 20110002029; 20110182587; 20110234435; 20120212360; 20120213531; 20130062508; 20130315597; 20130328706; 20140067300; 20150323852; and 20160087716.
A number of fast optical detectors and modulators have been developed for cryogenic environments, including both semiconducting and superconducting photosensitive elements. Semiconductor devices include a metal-semiconductor-metal (MSM) diode, and low-temperature-deposited GaAs, and generally function by increasing the density of charge carriers. Other novel materials such as graphene may also be used at cryogenic temperatures. See, for example, Phare, “Graphene electro-optic modulator with 30 GHz bandwidth”, Nature Photonics, vol. 9, p. 511 (2015).
Superconducting optical detectors include Josephson junctions, ultrathin niobium nitride films (NbN), superconducting tunnel junctions, and transition-edge sensors. The superconducting devices can be configured to be quite sensitive to weak optical intensities, with output signals that are well matched to superconducting readout circuits. Detection mechanism may include nonequilibrium heating of the superconductor (see Ilin, “Picosecond hot-electron relaxation in NbN superconducting photodetectors”, Applied Physics Letters, vol. 76, p. 2752, 2000), or altering the conductance of a Josephson junction or tunnel barrier (see Andreozzi, “Tunneling characteristics of Pb—CdS—Pb light-sensitive Josephson Junctions,” IEEE Trans. on Appl. Supercond., vol. 19(3), p. 983, 1983). The critical current of an element may change, or its kinetic inductance, or its resistance, in a transient manner that recovers quickly, on the 100 ps timescale or faster. Sensors may be sensitive to the signal of a single photon, particularly for photons in the infrared or visible range. Detectors may also provide spectral information, i.e., determine the energy of a single photon. See, for example, the following U.S. Pat. Nos. 6,812,464; 9,500,519; 2014/0353476; 6,815,708; 5,039,951; 5,057,485; 5,880,468; 6,239,431; 8,761,848; 9,577,176; 9,523,777; also JP 5,158,920.
There have been several systems for using optical pulses to generate SFQ pulses. See, for example, Kaplounenko, U.S. Pat. No. 5,963,351, “Digital Optical Receiver with Instantaneous Clock Recovery Circuit”, issued 1999; Sobolewski, “Ultrafast optoelectronic interface for digital superconducting electronics”, Superconductor Science and Technology, vol. 14, pp. 994-1000 (2001); Shinada et al., “1550 nm band optical input module with superconducting SFQ circuit”, Applied Physics Letters, vol. 96, 182504 (2010). In these systems, the optical intensity is not measured, but the detector serves just as an on-off switch.
Another type of readout scheme for a superconducting sensor that provides intensity information is to couple it to a superconducting resonator. If the optical signal causes an inductance, capacitance, or resistance of such a resonator to change, this will alter the spectral response of the resonator. Alternatively, a superconducting quantum interference device or SQUID has also been used in the prior art as an output device, generally as a low-noise analog amplifier. While a SQUID may also be used as a fast digital device, this mode has not been reported in connection with optical sensors. See Chevernak, “Superconducting multiplexer for arrays of transition-edge sensors”, Applied Physics Letters, vol. 74, p. 4043 (1999); Mazin, “Digital readouts for large microwave low-temperature detector arrays”, Nuclear Instruments and Methods in Physics A, vol. 559, p. 799 (2006). While these systems disclose conversion to digital signals, this is implemented in a separate digital processing system not integrated with the detection. See, Mazin, B. A., Day, P. K., Leduc, H. G., Vayonakis, A. & Zmuidzinas, J. Superconducting kinetic inductance photon detectors. Proc. SPIE 4849, 283-293 (2002). citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.562.6076&rep=rep1&type=pdf
Thus, there have been no reported multi-bit digital integrated readout schemes of fast superconducting optical sensors on the multi-GHz timescale.
There have also been efforts to provide a modulated optical output from digital superconducting devices (digital-to-optical conversion), but this has been very difficult to implement due to the severe mismatch in voltage and energy levels, so these have not been reduced to practice. Several US Patents that address this include the following: U.S. Pat. Nos. 6,661,560; 6,476,956; 5,886,809; 5,566,015; 5,110,792 2002/0105948. Unpublished application Ser. No. 15/356,030 also addresses this issue. Superconducting digital-to-optical conversion is not a subject of the present invention.
What is needed is a fast analog-to-digital converter that is integrated with a fast cryogenic optical demodulator, for converting a broadband optical signal to a multi-bit digital broadband superconducting signal.